In this chapter is discussed a miscellany of flagellates, most of which are found in the digestive tract. Only a few are pathogenic, the great majority being commensal. Some are not parasitic at all but are coprophilic or have been found as contaminants in washings from the sheath of bulls; these are mentioned because they must be differentiated from parasitic forms. A few other species are free-living toxin-producers.
Toxic Marine Phytoflagellates
The great majority of phytoflagellates are free-living and holophytic. Some of them produce powerful toxins which may kill fish or even man. Gonyaulax catanella is a marine dinoflagellate, found particularly off the coast of California, which causes a fatal disease of man known as mussel poisoning. Its toxin is one of the most powerful known. Under conditions still largely unknown, the protozoa multiply tremendously, forming a luminescent "bloom" in the ocean. Mussels and certain other shellfish feeding on plankton are not harmed by the toxin but accumulate it in their internal organs. People who eat these mussels may then be killed by the toxin.
The blooming of other dinoflagellates, including several species of Gymnodinium, cause the "red tide" or "red water" which sometimes kills huge numbers of fish, depositing them in rotting windrows on the shore. This condition is particularly common off the coast of Florida, where it is associated with the discharge of phosphates into the ocean, but it also occurs off the Texas coast and elsewhere (Hutner and McLaughlin, 1958).
A third marine phytoflagellate, the chrysomonad Prymnesium parvum, has killed fish en masse in brackish fish ponds in Israel, and has formed blooms accompanying fish kills in Holland and Denmark (Me Laughlin, 1958).
A few phytoflagellates are coprophilic and may be mistaken for true parasites, and still fewer are parasitic. These will be discussed below.
The parasitic mode of life has arisen independently a number of times in this group. Free-living species or genera in distantly related or unrelated families have found suitable living conditions in various hosts. Many of these were previously inhabitants of stagnant water. The fact that so few of them are pathogenic effectively refutes the notion that parasites tend to be pathogenic in their first association with a new host and that later on the host and parasite adjust to each other, the latter becoming less pathogenic and eventually commensal.